A fuel processing system often consists of many different components that all contribute to the production of a low carbon monoxide, hydrogen containing feed stream. The most common configuration of a fuel processing system employs auto thermal reforming or steam reforming in a Primary Reactor where air and/or steam are used for incomplete oxidation of the fuel to form H2, N2, CO, CO2, CH4 and H2O. A water gas shift reactor and a preferential oxidation reactor then utilize steam and air in conjunction with the primary reactor effluent for oxidation of carbon monoxide to produce stack-grade hydrogen. Until stack-grade hydrogen is produced, effluent from the preferential oxidation reactor is diverted to a tail gas combustor wherein any remaining fuel products are completely oxidized.
A limiting factor for the initial production of stack-grade hydrogen from a fuel processing system is the ability to raise each component up to its operating temperature. The heat required to accomplish this task can be related to the mass of each component's active material or reactor bed; the heat capacity of each component as determined by the size and materials utilized therein; and the temperature rise required within each component from the ambient temperature to the desired operating temperature range.
Conventional fuel processing systems use a cascading heating scheme for start up. This scheme usually involves heating the primary reactor up to a threshold light off temperature followed by the introduction of a rich mixture of fuel and air, along with steam if available. This mixture is allowed to react up to its adiabatic temperature and the products are allowed to flow through the primary reactor, and then onto the remaining components in the fuel processing system—namely water gas shift, PrOx, combustor, etc. The fuel cell stack is bypassed in this mode, as the primary reactor products are high in carbon monoxide and all of the products are directed to the combustor where they are completely oxidized. The heat generated in the combustor is recovered to indirectly heat the components of the fuel processing system. Great efforts must be undertaken in the form of heat exchangers in order to transfer heat produced in the combustor to the other fuel processing components. In this scheme, the primary reactor pulls most of the heat of combustion out of the stream passing through until it reaches its operating temperature. During that period of time, little additional heat is available to raise the temperature of the downstream components. This progression continues down the stream of components where each reactor is heated in sequence until the whole system reaches an operating temperature.
In this scheme, the amount of fuel that can be burned in the primary reactor to provide heat for the fuel processing components is limited by the sizing of the combustor. During normal operation (i.e. run mode), the fuel cell stack typically consumes 80%-90% of the hydrogen generated by the fuel processing components. This means that the combustor, during normal operation, should be sized to consume 10%-20% of the hydrogen produced. Therefore, without utilizing an oversized combustor, the heating of the fuel processor cannot exceed approximately 20% of full power.
Accordingly, the start up methods for conventional fuel processing systems are inadequate in that the sequential heating of the components increases the length of time necessary to reach a normal operating condition. Accordingly, there is a need in the art to provide a method for decreasing the start up time of a fuel processing system.